It is known to visualize the telephone call signal at the station of subscribers. This is particularly advantageous for deaf subscribers, for subscribers' stations located in noisy surroundings or, for example, in locations where numerous telephone receivers are located in a small area, for example in offices where closely located desks each are provided with telephones which ring frequently.
It has also been proposed to provide optical indication of signals which, usually, provide audible output, for example such as annunciators, doorbells, or other ringing devices.